Pathogenic microbes, molds, mildew, spores, and organic and inorganic pollutants are commonly found in the environment. Microbial control and disinfection in environmental spaces is desirable to improve health. Numerous ways have been used to in the past in an attempt to purify air and disinfect surfaces. For example, it is already known that Reactive Oxidizing Species (ROS) produced by, e.g., photocatalytic oxidation process can oxidize organic pollutants and kill microorganisms. More particularly, hydroxyl radical, hydroperoxyl radicals, chlorine and ozone, end products of the photocatalytic reaction, have been known to be capable of oxidizing organic compounds and killing microorganisms. However, there are limitations to the known methods and devices, not only due to efficacy limitation but also due to safety issues.
ROS is the term used to describe the highly activated air that results from exposure of ambient humid air to ultraviolet light. Light in the ultraviolet range emits photons at a frequency that when absorbed has sufficient energy to break chemical bonds. UV light at wavelengths of 250-255 nm is routinely used as a biocide. Light below about 181 nm, up to 182-187 nm is competitive with corona discharge in its ability to produce ozone. Ozonation and UV radiation are both being used for disinfection in community water systems. Ozone is currently being used to treat industrial wastewater and cooling towers.
Hydrogen peroxide is generally known to have antimicrobial properties and has been used in aqueous solution for disinfection and microbial control. Attempts to use hydrogen peroxide in the gas phase, however, have previously been hampered by technical hurdles to the production of Purified Hydrogen Peroxide Gas (PHPG). Vaporized aqueous solutions of hydrogen peroxide produce an aerosol of microdroplets composed of aqueous hydrogen peroxide solution. Various processes for “drying” vaporized hydrogen peroxide solutions produce, at best, a hydrated form of hydrogen peroxide. These hydrated hydrogen peroxide molecules are surrounded by water molecules bonded by electrostatic attraction and London Forces. Thus, the ability of the hydrogen peroxide molecules to directly interact with the environment by electrostatic means is greatly attenuated by the bonded molecular water, which effectively alters the fundamental electrostatic configuration of the encapsulated hydrogen peroxide molecule. Further, the lowest concentration of vaporized hydrogen peroxide that can be achieved is generally well above the 1.0 ppm OSHA workplace safety limit, making these processes unsuitable for use in occupied areas.
Photocatalysts that have been demonstrated for the destruction of organic pollutants in fluid include but are not limited to TiO2, ZnO, SnO2, WO3, CdS, ZrO2, SB2O4 and Fe2O3. Titanium dioxide is chemically stable, has a suitable bandgap for UV/Visible photoactivation, and is relatively inexpensive. Therefore, photocatalytic chemistry of titanium dioxide has been extensively studied over the last thirty years for removal of organic and inorganic compounds from contaminated air and water.
Because photocatalysts can generate hydroxyl radicals from adsorbed water when activated by ultraviolet light of sufficient energy, they show promise for use in the production of PHPG for release into the environment when applied in the gas phase. Existing applications of photocatalysis, however, have focused on the generation of a plasma containing many different reactive chemical species. Further, the majority of the chemical species in the photocatalytic plasma are reactive with hydrogen peroxide, and inhibit the production of hydrogen peroxide gas by means of reactions that destroy hydrogen peroxide. Also, any organic gases that are introduced into the plasma inhibit hydrogen peroxide production both by direct reaction with hydrogen peroxide and by the reaction of their oxidized products with hydrogen peroxide.
The photocatalytic reactor itself also limits the production of PHPG for release into the environment. Because hydrogen peroxide has greater chemical potential than oxygen to be reduced as a sacrificial oxidant, it is preferentially reduced as it moves downstream in photocatalytic reactors as rapidly as it is produced by the oxidation of water.